In prior art, vinyl chloride is polymerized by using a suspension method, and organic catalysts, such as peroxides, azo compounds or protective colloids, such as gelatin, methylcellulose and polyvinyl alcohol, are commonly used in the process. The quality of the suspension polymer prepared according to the existing technical procedures does not sufficiently satisfy the production and application engineers, especially in terms of the thermal stability, and a large amount of heat stabilizer is required during the modification process of polyvinyl chloride (PVC), wherein the heat stabilizer is predominantly a lead-containing product.
In fact, the main ingredients (vinyl chloride VCM, water, auxiliaries) added for the standard suspension PVC polymerization process has been foreseen prior to the start of the polymerization. At the start of the polymerization reaction, a sufficient amount of initiator is required to be added to activate the reaction and continue to be added until the reaction is completed within the predetermined procedure time. The most commonly used initiators are organic peroxides (different types, various properties) with good effects, but at the same time, some of the key defects will also affect the processing performance and quality of the finished PVC.
The suspended polyvinyl chloride (S-PVC) in the industry is currently and mainly produced by suspension polymerization via a batch feed process using a stainless steel autoclave. VCM is polymerized in accordance with the mechanism of free radical, and monomers are added into the reaction autoclave as liquid form. The water phase includes desalted water, dispersants (protective colloids, surfactants), buffer salts, and usually further includes defoamers, the water phase are finally dispersed into small droplets by continuous stirring.
After the initiator is added, the polymerization reaction is started, and the initiator is dissolved in the organic VCM phase. During polymerization, the medium must be continuously stirred. Thus, the polymerization is carried out only between VCM droplets. The reaction is carried out at a constant temperature of 35-75° C. under saturated vapor pressure of VCM. Eventually, the polymerization of VCM ceases after the addition of the killing agent capable of capturing free radicals. In general case, according to the production configuration and operating characteristics, the average conversion rate between VCM and finished PVC is 82-86%. Moreover, killing agents are a class of chemical products at higher prices.
The morphology of suspended PVC and the microstructure of S-PVC particles depend primarily on the polymerization temperature, the types and amounts of dispersants, the type of stirrer and stirring conditions.
The types of dispersants used also greatly determine the morphology of the PVC, because the dispersants can prevent the aggregation of the monomer VCM droplets during the polymerization reaction. Commonly used dispersants are partially water-soluble polymers such as cellulose derivatives and partially alcoholized polyvinyl alcohols (commonly referred to as PVAs). PVAs with different alcoholysis degrees are widely used and commonly can be used for commercial production of S-PVC. The alcoholysis degrees and molecular weights of PVAs determine their protective behaviors and the final sizes, shapes, and pores (morphology) of the polymer particles. The distribution of PVAs on the surface of VCM droplets depends on their alcoholysis degrees. The “primary” dispersant controls the size of polymer particles, and such PVAs are block copolymers having high alcoholysis degrees and having medium and high molecular weights, in general, their alcoholysis degree is generally more than 70%. On the other hand, the “secondary” dispersant mainly controls the pores of the polymer particles. PVAs serving as the secondary dispersant belong to random copolymers, having medium alcoholysis degrees generally in a range of 20-60%, with lower molecular weights.
Initiators commonly used in VCM polymerization reaction are generally different types of organic peroxides and azo compounds, solid long-chain hydroxy peroxydicarbonate salts, such as diacetoxy dicarbonates or perbis-myristates, which are mainly formulated into water suspensions and widely used due to the stability and ease of operation. A certain amount of initiator has high repeatability/reproducibility after being added to a reaction kettle, and because of its very low solubility in water, a hard shell would be formed mostly at the surface of the reaction kettle.
At present, an increasing number of solid peroxy decarbonate salts are replaced with liquid peroxy decarbonate salts. Liquid peroxy decarbonate salts are cheaper, and can dissolve in VCM more quickly. But there is a high risk of explosion, so the operation must be careful, which becomes a daily problem in the industry.
The performance of organic peroxides mainly depends on the decomposition rate, which is expressed by the half-life at a specific temperature. As described above, the two or more initiators of different types are often used in combination due to the difference in polymerization temperature.
When PVC having a high K value (K80 to K100) is to be produced, the polymerization temperature is below 50° C. Rapid initiators such as biisobutyryl peroxides are often used to enhance the slow initiation rates caused by low polymerization temperatures. Such initiators are often used in combination with slightly slower initiators such as biisobutyryl peroxides. Rapid initiators and peroxy decarbonate salts are often used after mixing for the PVC having a K value between 70-80.
S-PVCs of specification between K50-K70 are produced using peroxy decarbonate salts, neutral reactive peresters and diacyl peroxides or using these initiators in combination. In the initial stage of the initiation phase, the droplets of the initiator rapidly disappeared because the initiator diffused into the VCM droplets and then aggregated. From the initial stage of the polymerization process until the conversion rate of 15%, the monomer droplets are dispersed and aggregated again, such that the initiator was evenly distributed between the VCM droplets.
The distribution of the initiator on the VCM droplets would affect the morphology of the finished S-PVC particles. If the initiator is not evenly distributed on each VCM droplet, particles with no pores are formed, which are referred to as plasticized particles. These non-porous particles have vitreous properties and high densities, and are almost spherical. Compact vitreous particles are difficult to process. Due to the lack of pores, intakes of plasticizer and stabilizer are very little, resulting in that these particles are not easy to be gelatinized and moulded during the process of processing. The presence of these particles leads to visible defects at the surface of finished S-PVC, i.e., so-called “fish eye”.
The production of these nonporous vitreous particles originates from presence of the large amount of initiator droplets inside the polymerization reaction, replacing the conventional polymerization between VCM droplets. When the initiator is not sufficiently dispersed in the aqueous medium, the initiator droplets having a wide particle size distribution are formed. Under normal circumstances, the small initiator droplets will disappear quickly due to condensation with larger VCM droplets. However, the presence of larger VCM droplets is due to the fact that the fusion of smaller VCM droplets with these initiator droplets produces an opposite effect. The amount ratio of a small amount of VCM droplets and a large number of initiator droplets determines the polymerization change within the initiator droplets and the formation of these vitreous particles.
Another existing major problem is that these vitreous particles have poor thermal stability. This phenomenon is very likely to result from impact of the polymer chain with the initiator debris caused by the lack of monomer and thus leading to the formation of unsaturated structure.
The initiator residues present in the PVC would also affect the degradation of the polymer during processing. Under the condition of high temperatures required for the processing, active free radicals will form, and dehydrochlorinationis is carried out by extracting H— from the polymer chain, resulting in the initial coloring of PVC products. If the peroxide is easily hydrogenated by the water present in the polymerization system (mainly occurred during the later stage of the polymerization), the degradation of the PVC during processing will decrease due to the less residual amount of initiator in the material.